Transistors can be divided into two main types: bipolar junction transistors and field-effect transistors. Both types share a common structure comprising three electrodes with a semi-conductive material disposed therebetween in a channel region. Transistors can be formed by depositing the components in thin films to form thin film transistors. When an organic material is used as the semiconductive material in such a device, it is known as an organic thin film transistor.
Field-effect transistors are three-terminal devices which comprise a source contact, a drain contact, and a gate contact. The source and drain contacts are connected by a semiconductive layer (the channel), itself spaced from the gate contact by an insulating layer called the gate dielectric. In polymer transistors, the semiconductive layer is fabricated from a semiconductive polymer, typically a n-conjugated organic polymer. This layer may be deposited in the device by a precursor route or directly by solution-processing.
A voltage is applied across the source contact and the drain contact. Further, a voltage is applied to the gate contact. This voltage creates a field which alters the current-voltage characteristics of the semiconductive layer lying directly under the gate dielectric by causing accumulation or depletion of charge carriers there. This in turn modulates the rate at which charges pass through the channel from the source to the drain contact for a given source-drain voltage.
Therefore, for OTFTs the conductivity of the channel can be altered by the application of a voltage at the gate. In this way the transistor can be switched on and off using an applied gate voltage.
A typical organic light-emitting device comprises two layers of organic material, one of which is a layer of light emitting material such as a light emitting polymer (LEP), oligomer or a light emitting low molecular weight material, and the other of which is a layer of a hole transporting material such as a polythiophene derivative or a polyaniline derivative.
Organic LEDs may be deposited on a substrate in a matrix of pixels to form a single or multi-colour pixelated display, which can be switched between emitting and non-emitting states by altering the current flow through them using a FET. A multicolored display may be constructed using groups of red, green, and blue emitting pixels. So-called active matrix (AM) displays have a memory element, typically a storage capacitor and a FET, associated with each pixel whilst passive matrix displays have no such memory element and instead are repetitively scanned to give the impression of a steady image. Examples of polymer and small-molecule active matrix display drivers can be found in WO 99/42983 and EP 0,717,446A respectively.
An active matrix driven display may be either bottom-emitting or top-emitting. In a bottom-emitting display light is emitted through the substrate on which the active matrix circuitry is fabricated; in a top-emitting display light is emitted towards a front face of the display without having to pass through a layer of the display in which the active matrix circuitry is fabricated.
For good operation, the source-drain materials for OTFT devices must make ohmic contact with the organic semiconductor. Since OTFTs are typically p-channel devices with holes as the majority charge carriers, high work function metals must be used. Examples of standard materials that are used for this purpose are gold, platinum and palladium. While these metals have proved to be very useful, there are downsides to their use, including high cost and diffusion (particularly with gold). For this reason, some examples of source-drain contacts are modified at their surface by the inclusion of an additional layer of metal oxide such as MoO3, WO3 or V2O5. These types of materials have also been used as hole-injecting contacts for polymer light emitting devices, due to their good energy level matching with the HOMO of light emitting polymers.
US 2005/0263756 is concerned with an organic field effect transistor and a method of manufacturing the same. It is stated that it is an object of US 2005/0263756 to provide an organic field effect transistor in which element characteristics and stability are improved. An organic field effect transistor is disclosed including a gate electrode formed on an organic semiconductor film made of an organic semiconductor material with a gate insulating film interposed therebetween; and a source electrode and a drain electrode formed so as to come in contact with the organic semiconductor film with a charge injection layer made of an inorganic material interposed therebetween, and the organic semiconductor film holes serving as a majority carrier.
US 2005/0279999 relates to a thin film transistor. It is said that there is provided a thin film transistor, which makes electrical communication between source and drain electrodes efficient. There is provided a bottom gate thin film transistor including a substrate, a gate electrode provided on the substrate, a gate insulating layer provided on the gate electrode, and an organic semiconductor layer contacting the source electrode and the drain electrode and insulated from the gate electrode, wherein oxidation portions are provided in portions of the source electrode and the drain electrode that make contact with the organic semiconductor layer. During the method of manufacture of the bottom gate structure described in paragraphs [0034] and [0035], after forming the gate insulating layer, a source and drain electrode layer is formed. Portions of the source and drain electrodes that will, make contact with the organic semiconductor layer (formed later) are oxidised. The gate insulating layer is susceptible to damage by the oxidation process. This limits the choice of oxidation techniques available in this method.
Chu et al Applied Physics Letters 87, 193508 (2005) discloses bilayer source-drain electrodes for organic thin film transistors. The bilayer consists of a transition metal oxide layer (MoO3) and a metal layer. The metal oxide layer directly contacts the organic semiconducting layer and serves as a charge injection layer. The metal layer is coated thereover. The MoO3 of the examples is thermally evaporated directly onto a pentacene active layer film.
It will be appreciated from the above that OTFTs can have complicated structures with numerous layers in a desired layout. Consequently, making these structures involves numerous method steps and techniques. The present invention at least partially aims to simplify existing methods for making OTFTs.
Further, it will be appreciated from the above that there is a problem in this field to develop electrode materials having the necessary deposition and patterning characteristics in combination with the required injection characteristics. In particular, when using the same material for the source and drain electrodes, the material must form good ohmic contacts (achievable by workfunction matching) in the device in order to act as a charge extractor and injector. A further aim of the present invention is to at least partially address this problem.